1. Field of the Invention
The present invention relates to a dispersion compensating method and a dispersion compensating apparatus, and, in particular, to a dispersion compensating method and a dispersion compensating apparatus for automatically compensating wavelength dispersion occurring in an optical fiber in a wavelength division multiplexing transmission system.
2. Description of the Related Art
Recently, a transmission rate in an optical transmission system increases from 2.4 Gbps to 10 Gbps, and, in such a situation, importance of dispersion compensation increases accordingly.
In a wavelength division multiplexing (WDM) transmission system, there are two methods of dispersion compensation, i.e., a method in which dispersion compensation is performed for each wavelength and another method in which dispersion compensation is performed for all the wavelengths in a lump. The first method in which dispersion compensation is performed for each wavelength requires the costs greatly higher than those required for the second method in which dispersion compensation is performed for all the wavelengths in a lump. On the other hand, since dispersion slope in a transmission line depends on each particular optical fiber transmission line, it is not possible to completely compensate transmission line dispersion for all the wavelengths in the second method of performing dispersion compensation in a lump of all the wavelengths. In order to solve this problem, a method of dividing a target wavelength band and performing dispersion compensation for each of the division wavelength bands exists.
In many cases, the above-mentioned second method of performing dispersion compensation in a lump is applied in terms of the costs required. However, in any method, a dispersion compensator may be used there is such that an optical fiber such as a dispersion compensated fiber (DCF) as a typical example is cut for a length according to a dispersion amount, and is disposed according to a transmission line dispersion amount for each repeating section, so as to be used for the dispersion compensation.
In contrast thereto, recently, a variable dispersion compensator has been put into practical use, and, it is expected that such an apparatus will spread sharply. It is known that, in order to control the variable compensator, an optimum dispersion amount is set with the use of code error information such as error correction amount as disclosed by Japanese Laid-open Patent Application No. 2002-208892.
Other than it, Japanese Laid-open Patent Application No. 2003-46443 discloses an art of detecting a duty ratio of an optical signal, and controlling a dispersion compensation amount in the variable compensator based on the duty ratio.
Further, Japanese Laid-open Patent Application No. 2000-244394 discloses an art of detecting maintainability in data transmitted, generating an electric feedback signal for controlling a current applied to a grating, and determining dispersion in the grating.
International Publication No. WO99/048231 discloses an art of giving perturbation to wavelength dispersion in a transmission line, counting and comparing errors occurring when the perturbation is given in each direction, and changing the center of the perturbation in a direction such that the errors may be reduced.